In optical transmission systems, knowing dispersion in optical transmission paths serving as optical-signal transmission media is useful and/or necessary for operation of the systems. The dispersion is caused by variations in a group velocity which depend on wavelength components of transmission wavelengths, when signal light is transmitted through an optical transmission path. The dispersion causes waveform degradation at the receiving end. One example of an available method for measuring dispersion in an optical-fiber transmission path is a method in which two optical pulses with different wavelengths are simultaneously launched into an optical fiber and the propagation time of the optical pulses that have passed through the optical fiber transmission path are measured.
Dispersion DL in an optical fiber for a wavelength λ1 and a wavelength λ2 is determined from the wavelengths of optical pulses propagating in the optical fiber and the propagation time thereof, in accordance with:
                    DL        =                                            Δ              ⁢                                                          ⁢                              t                1                                      -                          Δ              ⁢                                                          ⁢                              t                0                                                                        λ              2                        -                          λ              1                                                          (        1        )            where Δt0 denotes a time difference between two optical pulses at the transmitting end and Δt1 denotes a time difference between the two optical pulses at the receiving end.
FIG. 1 illustrates an example of dispersion measurement. In the example of FIG. 1, the transmitting end launches optical pulses (e.g., rectangular waves) with wavelengths λ1 and λ2 and with a pulse width w into an optical fiber at the same time (Δt0=0) and the receiving end receives the optical pulses through the optical-fiber transmission path. In this case, letting Δt1 be the time difference between the two optical pulses at the receiving end, the dispersion in the optical fiber is given by equation (1) noted above.
Examples of related art include Japanese Laid-open Patent Publication No. 2008-64683 and Japanese Laid-open Patent Publication No. 2003-98037.
Hitherto, rectangular waves have been used as optical pulses for transmission signals for dispersion measurement. When the difference in propagation time during dispersion measurement is not sufficiently larger than the pulse width (indicated by w) of the rectangular waves, two optical pulses overlap each other, thus making it difficult to measure the difference between the propagation times of the two optical pulses. When the pulse width w of the rectangular waves is reduced, dispersion itself of the optical transmission path causes distortion of the waveform of the optical pulses, thereby making it difficult to measure the difference between the propagation times. In addition, a reduction in the optical pulse width w of the rectangular waves causes the S/N (signal to noise) ratio to decrease. Hence, optical pulses with a small pulse width w are not suitable for measurement of the dispersion in a transmission path that causes a large amount of dispersion.
Since an increase in the pulse width w of the optical pulses improves the S/N ratio, optical pulses with a large pulse width are suitable for measurement of dispersion over a long distance, but make it difficult to perform accurate measurement.
There has also been a method for separating signals for individual wavelengths by using an optical filter at the receiving end. This method, however, requires multiple receivers, thus increasing the overall size of the apparatus.
FIG. 2 illustrates an example of a case in which a difference between propagation times during dispersion measurement is smaller than the rectangular-wave-pulse width of optical pulses. In the example of FIG. 2, the transmitting end launches optical pulses (rectangular waves) with wavelengths λ1 and λ2 and with a pulse width w into an optical path at the same time (Δt0=0) and the receiving end receives the optical pulses through the optical-fiber transmission path. In this case, since the time difference Δt1 between the two optical pulses at the receiving end is smaller than the pulse width w of the optical pulses, it is difficult to measure the time difference. Thus, the dispersion in the optical fiber cannot be determined with accuracy.
FIG. 3 illustrates an example of a case in which the pulse width of the rectangular waves of the optical pulses is small during dispersion measurement. In the example of FIG. 3, the transmitting end launches rectangular-wave optical pulses with wavelengths λ1 and λ2 and with a small pulse width w into the optical fiber at the same time (Δt0=0) and the receiving end receives the optical pulses through the optical-fiber transmission path. The reduction in the pulse width increases a dispersion influence on the optical pulses and causes distortion of the waveform, thus making it difficult to measure the time difference between the two optical pulses.